Recently, there has been an increasing thrust in the application of internal antennas in wireless communications devices. The concept of an internal antenna stems from the avoidance of using an external radiating element through the integration of the antenna into the communications device itself. Internal antennas have several advantageous features such as being less prone to external damage, a reduction in overall size of the communications device with optimization, and easy portability. In most internal antennas, the printed circuit board of the communications device serves as the ground plane of the internal antenna.
With the advent of mobile communications devices capable of operating in more than one band, designers have begun to use separate antennas in conjunction with a switching unit wherein each antenna operates in a distinct frequency band. The switching unit selectively connects a transceiver of the communications device to one of the antennas. The conventional dual-band antennas, however consume a large amount of power and are known to have high manufacturing costs.
The foregoing concerns become even more pronounced where a communications device is required to operate in multiple radio applications such as, e.g., WiFi, Bluetooth and GPS applications. In particular, a significant challenge arises in terms of high coupling when a dual-feed antenna is implemented for operating at the same frequency band in a compact device such as a mobile communications device where stringent form factor and footprint requirements are typically the norm. Relatedly, high coupling between the feed ports can give rise to decreased radiation efficiency of the antenna as well.
In addition, current antenna solutions for Multiple Input Multiple Output (MIMO) applications require multiple antennas, which can result in duplication of certain parts of to build the communications device, thereby necessitating usually unfavorable trade-offs between device size and performance. Such trade-offs can be that smaller devices may suffer performance problems, including shortened battery life and potentially more dropped calls, whereas devices with better performance require larger housings. In general, the driver of this trade-off is mutual coupling between the antennas, which can result in wasted power when transmitting and a lower received power from incoming signals. In MIMO technologies such as Long Term Evolution (LTE), where two receive antennas are required, such cross-coupling effects can be highly undesirable since effective MIMO performance requires relatively low correlation between each of the received signals of the multiple antennas. Currently, this is typically accomplished in large devices using one or more of: spatial diversity (distance between antennas), pattern diversity (difference between antenna aiming directions), and polarization diversity. Unfortunately, when multiple antennas are used within a mobile handheld device, the signals received by each of the antennas are undesirably correlated, due to the tight confines typical of the compact devices that are favored by consumers. This noticeably disrupts MIMO performance. The trade-off is then to either enlarge the device, which may not be well received by the consumers, or else tolerate reduced performance.